The first thing about Mrs Kong is that it works. Reading the synopsis and text, it doesn't look that way. There is a plot you can't quite grasp, acted out by walking ideas who appear to have learnt their English from a Tippett libretto. Given that this bizarre mix of high-flown imagery and outdated vernacular was to be set by a master of the convoluted musical line, a heavy evening was in prospect.

Yet on stage the whole apparatus of Harrison Birtwistle and Russell Hoban's new opera comes brilliantly to life. Design and pacing do their tricks; the musicians deliver at full stretch. And you can extract a succinct story. The real Kong, the spirit that endures in the body of the giant ape, wants the girl with the pearl earring that Vermeer painted. With a bravery and stupidity to match Siegfried's, he goes in quest of her. She, passive and even dimmer, wants him too. But they can't get it together. Moral: dream on.

Of course it is subtler than that. In some two hours' action there is time to explore the nature of memory and obsession, to examine the connection of idea, image and reality, to struggle with the meaning of popular icons and even to take a few sideswipes at recent commercial culture. But the metaphysics falls into place as overtones and echoes, discontinuous lives and times connect with the dreamlike inevitability of a pre-Freudian fairy tale.

All credit to Tom Cairns' staging. Poetic and technological imagination coincide in a virtuoso feat of combined design and direction. Transparent projections and walk-through boxes create a tangible sense of separate historical times working simultaneously, and easily clarify the libretto's separate layers. With a gallery of unlikely characters to bring to life - literally, since they are supposed to be dead - Cairns exploits a flair for strong colour and telling costume to turn the gun-toting cowboy of a film producer or the Bhagwan-style 'spiritual adviser' from concepts into strong, individual characters.

And Hoban's words prove to be well judged for the purpose. The jokes, for one thing, are actually funny, while his larger- than-life turns of phrase supply the emotional force and spirit of parody that fuel the drama. The music comes next, and only next. Some of the stronger scenes - Kong and Pearl talking by computer across the centuries, the guru fighting the cowboy, the wisecracking sphinx - would work with almost any music.

Birtwistle gets off to a sticky start, setting the scene in dark, growling timbres but then keeping the tempo, and the temperature, cautious for a dangerously long time. The first scene also includes his well- worn device of replaying an action several times over. Is this self-parody? Soon, the invention catches fire with Pearl's fragile vocal line, Kong's cry of anguish, and later a spectacular accompaniment to the original film of Kong falling from the Empire State Building which matches the epic navety of the music for the horse in Gawain. But at a first encounter, even though it is not meant to be on the same grand scale as the earlier opera, the score does not seem to be driving the action with the same consistent power.

There's a problem with the end, and not in the music, which times the climax and wind-down just right. Pearl and Kong are supposed to stay apart, but Hoban lets them get to the starting-line. The failure then looks like Kong losing will, or Pearl not being attracted, or both. Is he a wimp? Is she a racist? Is the opera suddenly about sexual denial (like Parsifal, or Gawain), or merely making people confront the truth about themselves (moral: grow up)? Why does Inanna, who on paper looks like an object of scorn, get all the best lines and draw our sympathy when it is supposed to be going to Kong? Instead of fruitful ambivalence, it's confusion that reigns.

As you wonder, you can relish a gallery of superb performances, from Philip Langridge's despairing, resourceful Kong to Phyllis Cannan's show-stealing Inanna. Helen Field's sinuous soprano elevates Pearl out of her inbuilt stupor; Nuala Willis puts in a sharp cameo as Madame Lena, the sphinx; Michael Chance makes Orpheus (oh yes, he's back) sing his heart out. At Glyndebourne, the most breathless hush descended for the clips of King Kong itself. But then, isn't that the point?